more F move (trunk only)

Using F prefix when trying to move into a wall or closed door yielded
"you attack thin air".  Like the recently fixed F-vs-boulder case, give
more appropriate feedback.  Also like F-vs-boulder, initiate digging if
wielding a pick-axe.  (Also handles axes versus trees and closed doors).

     One thing which isn't handled but possibly should be:  F vs closed
door when not wielding a pick or other axe might attempt to force the door.
(Right now it gives "you harmlessly attack the door".)

include/extern.h

@@ -259,6 +259,7 @@ E void NDECL(sokoban_detect);
 
 /* ### dig.c ### */
 
+E int FDECL(dig_typ, (struct obj *,XCHAR_P,XCHAR_P));
 E boolean NDECL(is_digging);
 #ifdef USE_TRAMPOLI
 E int NDECL(dig);